Tuesday, July 14, 2020

Shirahama Castle -History of peninsular brave (1) Mysterious landing to peninsula-

Shirahama Castle

-History of peninsular brave (1) Mysterious landing to peninsula-



Name: Shirahama castle (Shirahama-jo)
Place: Shirahama Minami-Boso city, Chiba
Location: 34.91314276471758, 139.8894262284239
Type: Mountain Castle
Built: 15th century?
Remaining remnants: Clay walls and dry moats 

Brief History

Shirahama castle (白浜城) is placed over Shiroyama mountain, one of about 120 meter height from hillside at the north of former Shirahama town central of current Minami-Boso city. Shirahama area is the south end of Boso peninsula, and Nojimazaki Cape which is the southernmost point of the peninsula is just 1 kilometer south of Shiroyama mountain.

Shirahama town is a small one spreads over narrow flat area of 2 kilometer long and 500 meter wide, but from the maritime viewpoint which is an important point divides outer sea shipping of Pacific Ocean and inner sea shipping of Tokyo Bay. The fact that lighthouse was built at Cape Nojimasaki as one of the eight lighthouses built in 1866 just after opening the nation shows this fact.

Being supported by moderate rocky coast line which is suitable for small port, line of mountains in north, Nojimasaki cape at west and small island at south which might be used as shelter from winds and waves, linear location from the tip of Izu peninsular to Oshima island and Shirahama area, in spite of remoteness in land transportation, Shirahama area had good maritime communication from neighbor area, and this also brought Satomi clan to this area.

Origin of Satomi clan

The origin of Satomi clan is said as a branch family of Nitta clan, the local samurai of Nitta area of Kozuke province (Gunma prefecture). Nitta clan was a high class clan of Minamoto family, which is equivalent to Ashikaga clan, another high clan at Ashikaga area of Shimotsuke province (Tochigi prefecture), next to Minamoto clan itself.

At the establishment of Kamakura Shogunate, both clans supported Yoritomo Minamoto (1147-1199), the leader of main family of Minamoto clan and founder of Kamakura Shogunate. But after the fall of Kamakura Shogunate, Ashikaga clan established their own Muromachi Shogunate, and Nitta clan supported the South Court which was opponent of Muromachi Shogunate.

Nitta clan was ruined during the battle against Muromachi Shogunate. Major part of Satomi clan accompanied with Nitta clan, but small part belonged to Muromachi Shogunate and survived as small local lord. Muromachi Shogunate which stabilized its situation placed their relative at Kamakura city as Kamakura Kubo Highness, as a representative in Kanto region.

Conflict of Kamakura Kubo and Kanto Kanrei

Muromachi Shogunate also placed Uesugi clan as Kanto Kanrei, the general minister of Kamakura Kubo Highness, to support and restrain Kamakura Kubo Highness. As expected, Kamakura Kubo Highness which wanted to leave from Shogunate and Kanto Kanrei that tried to stop it fell into severe conflict.

During this conflict, Satomi clan belonged to Kamakura Kubo Highness, considering its fame as a traditional family of Minamoto clan common to Kamakura Kubo Highness. In 1438, the 4th Kamakura Kubo Highness Mochiuji Ashikaga (1398-1439) finally raised his army against Kanto Kanrei Norizane Uesugi (1416-1466) and Muromachi Shogunate.

Shogunate decided Mochiuji as the enemy of the Imperial Household, and send reinforcement army from neighbor provinces to rescue Norizane Uesugi. Mochiuji lost the battle became monk but finally was forced to suicide. At this time the leader of Satomi clan is said to follow their master Mochiuji Ashikaga. This is called as the battle of Eikyo, and the dominance of Kamakura Kubo Highness had ended.

Continued battle between two powers

However in 1440, remaining retainers of Mochiuji and Yuki clan which disliked interference of Muromachi Shogunate to Kanto region gathered under survived two sons of Mochiuji and besieged at Yuki castle (Ibaraki prefecture). After half year battle Yuki castle fell, and two sons of Mochiuji were killed but Shigeuji Ashikaga (1438-1492) who was still an infant was rescued. Leader of Satomi clan also died in this battle.

To stabilize the situation of Kanto region, in 1447, Muromachi Shogunate placed Shigeuji Ashikaga as Kamakura Kubo again under the support of Uesugi clan. But in addition to unstable structure same as before, tension between local lords fought as friends and enemies did not disappear.

In 1454, Shigeuji Ashikaga assassinated Kanto Kanrei Noritada Uesugi (1433-1455) and raised his army again, which is named as the battle of Kyotoku. Shigeuji chased the army of Uesugi clan to the east half of Kanto plain, but his main base Kamakura city was captured by Imagawa clan, the governor of Suruga province (middle part of Shizuoka prefecture) and sent as a reinforcement army to Uesugi clan from Shogunate.

Importance of peninsula

Shigeuji Ashikaga who lost Kamakura city moved to Koga castle (Ibaraki prefecture) and named as Koga Kubo highness, then managed east half of Kanto plain on left bank of Tone-gawa river. On the other hand, Uesugi clan seized west half of Kanto plain on right bank of Tone-gawa river, then further divided into Yamanouchi Uesugi clan, the main family at north half and its branch family Ogigayatsu Uesugi clan at south half.

Among the conflict between Koga Kubo Highness and Uesugi clan, Boso peninsula was a buffer zone of both powers. For Awa province at the tip of the peninsula and Kazusa province at the middle, both of Uesugi clan and Kamakura Kubo related lords served the governor position in turn. Because of hilly terrain separated into small flat areas, local lords of both side remained together.

Considering the deadlock of main front line facing Tone-gawa river, Boso Peninsula might be a good route of detour attack for both side. The tip of the peninsula is just 30 kilometer apart from Kamakura city which was former main base of Kamakura Kubo Highness across Uraga-Suido straight, and the root of the peninsula was backward of Koga castle, the main base of Koga Kubo Highness.

Landing to Boso peninsula

Around 1460, Koga Kubo Highness sent their close retainer Nobunaga Takeda (?-1477) to Kazusa province, and Yoshizane Satomi (1412-1488) to Awa province respectively, to seize whole part of Boso peninsula. As Uesugi clan had their base at current Shirahama area, the south edge of the province, Yoshizane Satomi chose Shirahama area as landing place. Now the progress of Satomi clan toward the peninsular brave over 100 years started.

At this point, Awa province was separately governed by strong local lords such as Anzai clan at northwest, Kanamari clan at Tateyama area, Tojo clan at Kamokawa area. For Shirahama area, Uesugi clan directly managed the area and placed their magistrate Kiso clan at the area.

As the record of the beginning of Satomi clan was lost due to later coup d’etat, the process is unknown but Yoshizane Satomi could seize Shirahama area, being supported by the turn of Kiso clan from Uesugi clan to Satomi clan. Yoshizane captured Shirahama area might build Shirahama castle as his main base in Awa province.

Structure of Shirahama castle

Shirahama castle spreads over the edge of coastal terrace runs east and westward over 1 kilometer. As southern slope of the edge is a sheer cliff, the castle places core areas along the edge in line and built terraces over ridges spread northward.

Climbing from Aoki-Kannondo shrine, there becomes a saddle point separated the castle into west half and east half. West half is core area of the castle, and the peak of the mountain at the west of the saddle point might be the central area of the castle. There are many terraces around the core area but no clear plan and facility for defense.

According to the restore diagram of the castle, there are also many terraces in east half of the castle, and total length of the castle exceeds 800 meter. This size is too much considering small size of Satomi clan at the start, then this castle might be expanded for the evacuation space of residents preparing for the attack of the enemy. 

Afterward of castle

Yoshizane Satomi captured Shirahama area next aimed Tateyama area, which has the largest flat land in the province with cultural production. After short time, Satomi clan entered Tateyama area and built next main base Inamura castle (Chiba prefecture), at the backward of Tateyama plain and center of Awa province. Now Satomi clan becomes the lord of whole Awa province.

Once Satomi clan united Awa province, there was no imminent military operation for Shirahama area at the south edge, except for maritime attack from the sea. Shirahama castle might be kept but had no necessity for development, thus immature structure of the castle might be kept. The castle might be used by 1590, the end of Sengoku-era.

Now west half of castle becomes a hiking road, but because of the collapsed muddy ground it is hard to walk. And it is accessible only around hiking road, and major part of the ruin is covered by heavy bush. The structure of the castle is quire mysterious due to low accessibility, and this is also applicable to mysterious story of early days of Satomi clan with quite limited record.


45 minutes drive from Futtu-Tateyama Doro expressway Tomiyama interchange.

Related Castles

Inamura Castle -History of peninsular brave (2) Coup d'etat erased from history-
Koga Castle -Castle disappeared into embankment-

Pictures (click to enlarge)

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